The most common answer:

\[ \frac{1}{n} \sum_{i = 1}^{n} (x_i - q_{.5})^2 \]

Other creative answers:

• "modal range": \[ mode_{upper\,quartile} - mode_{lower\,quartile}\]
• meta-variance: take the mean of the variance, sd, IQR, and range
• look at the max height of the density plot